A conventional dock leveler has a deck assembly which stores horizontally and level with the dock floor. It has a pivoting lip which extends outward to rest on the vehicle which is being loaded. The lip provides the bridge between the vehicle and the leveler deck to allow material transfer operations to occur. When the dock leveler is stored, the lip pivots to a pendant position. Usually the end of the lip rests in lip keepers on the stationary frame and supports the front of the deck assembly in the stored position. There are many different systems used to control the motion of the lip, from mechanical linkages to hydraulic cylinders. As examples of systems used to control hydraulic dock levelers, reference is made to U.S. Pat. Nos. 4,081,874; 4,744,121; 4,955,923 and 5,205,010.
This invention relates to a dock leveler which uses hydraulic cylinders to control the position of both the deck assembly and the lip assembly. A typical dock leveler has a larger "main cylinder" to lift the deck assembly and a smaller "lip cylinder" to control the lip position. There are two common methods of controlling the lip, namely "gravity fall" and "powered-in". The more common method is "gravity fall" where the lip is held extended by the lip cylinder until the leveler rests on the vehicle. When the loading operation has been completed, the deck assembly is raised by the main cylinder and the lip falls by gravity to the pendant position. When operating conditions are ideal, the "gravity fall" lip works well. However, dock levelers operate in a harsh environment of industrial loading platforms where dirt, corrosion, ice or snow may prevent the lip from falling. If the lip does not fully retract to rest in the lip keepers, the leveler cannot be stored in the cross-traffic position.
If the dock leveler has an optional "automatic return" feature and the vehicle leaves the dock before the dock leveler has been restored, the deck assembly will fall by gravity to rest on the frame of the dock leveler. The power unit will then automatically start and lift the dock leveler until the lip is fully retracted, or until a preset time has elapsed. If the lip does not fully retract, the hydraulic unit may continue run indefinitely and cause overheating and damage to the unit.
The "powered-in" method is similar in operation to the gravity fall except the lip cylinder is "double acting". That is, when hydraulic pressure is applied to the piston side, the lip will extend and when pressure is applied to the rod side, the lip will retract. When the lip is being extended both methods operate the same. But when the dock leveler is being restored from the operative position with the lip resting on a vehicle, a "powered-in" hydraulic circuit will apply hydraulic pressure to the rod side of the lip cylinder and force the lip to retract, overcoming the friction of corrosion and foreign material in the lip hinge.
The disadvantage of the "powered-in" system is that the pressure of the lip trying to retract as the deck is raised from the bed of the vehicle can result in undesirable scuffing of the bed of the vehicle. This is of particular concern with trailers having an aluminum bed. The second problem is "lip droop", that is, the tendency of the lip to retract slightly from full extension each time the power unit stops. This may prevent proper placement of the lip on the vehicle bed. Lip droop is a consequence of the result of loss of hydraulic fluid from the secondary circuit caused by a time delay from the pump stopping to the time when the pilot-operated check valve has fully closed. Lip droop is evident in most hydraulic dock levelers, but the amount of droop is usually greater with the powered-in design because of the hydraulic pressure urging the lip cylinder to retract.